public RecordCountPerUnitCollector(
     final String sample, final String library, final String readGroup) {
   metric = new TotalNumberMetric();
   metric.SAMPLE = sample;
   metric.LIBRARY = library;
   metric.READ_GROUP = readGroup;
   unitsToMetrics.put(
       noneOrStr(sample) + "_" + noneOrStr(library) + "_" + noneOrStr(readGroup), metric);
 }